In the rat, activity-stress (A-S) treatment was related to varying degrees of inflammation, goblet depletion, and erosion of the mucosal wall. Young (aged twenty-eight days), and mature (aged seventy days), rats did not differ significantly in terms of overall thinning of the colon wall, however the incidence of hematuria, hemorrhage at fecal bolus sites, mucosal hyperemia and interstitial compression of the mucosal wall was greater for younger animals. These findings suggest that A-S produces both more severe and more acute colonic pathology in young animals. The use of the A-S paradigm as an appropriate route for inducing psychological stress is discussed in terms of its capacity to evoke a somatic stress response as indicated by stereotypical alterations of structure and function of the gastrointestinal and thymicolymphatic system.